104 research outputs found
Frequency warping compressive sensing for structural monitoring of aircraft wing
No full textThis work focuses on an ultrasonic guided wave structural health monitoring (SHM) system development for aircraft wing inspection. The performed work simulate small, low-cost and light-weight piezoelectric discs bonded to various parts of the aircraft wing, in a form of relatively sparse arrays, for cracks and corrosion monitoring. The piezoelectric discs take turns generating and receiving ultrasonic guided waves. The development of an in situ health monitoring system that can inspect large areas and communicate remotely to the inspector is highly computational demanding due to both the huge number of Piezoelectric sensors needed and the high sampling frequency. To address this problem, a general approach for low rate sampling is developed. Compressive Sensing (CS) has emerged as a potentially viable technique for the efficient acquisition that exploits the sparse representation of dispersive ultrasonic guided waves in the frequency warped basis. The framework is applied to lower the sampling frequency and to enhance defect localization performances of Lamb wave inspection systems. The approach is based on the inverse Warped Frequency Transform (WFT) as the sparsifying basis for the Compressive Sensing acquisition and to compensate the dispersive behaviour of Lamb waves. As a result, an automatic detection procedure to locate defect-induced reflections was demonstrated and successfully tested on simulated Lamb waves propagating in an aluminum wing specimen using PZFlex software. The proposed method is suitable for defect detection and can be easily implemented for real application to structural health monitoring
Compressive sensing with frequency warped compensation for damage detection in composite plate
No full textThis work focuses on an ultrasonic guided wave structural health monitoring (SHM) system development for composite plate inspection. The development of an in situ health monitoring system that can inspect large areas and communicate remotely to the inspector is highly computational demanding due to both the huge number of piezoelectric sensors needed and the high sampling frequency. To address this problem, a general approach for low rate sampling is developed. Compressive Sensing (CS) has emerged as a potentially viable technique for the efficient acquisition that exploits the sparse representation of dispersive ultrasonic guided waves in the frequency warped basis. The framework is applied to lower the sampling frequency and to enhance defect localization performances of Lamb wave inspection systems. As a result, an automatic detection procedure to locate defect-induced reflections was demonstrated and successfully tested on simulated Lamb waves propagating in a carbon fiber plate using PZFlex software. The proposed method is suitable for defect detection and can be easily implemented for real application to structural health monitoring
Nanodroplet-based ultrasound and photoacoustic super-resolution and functional imaging
No full textMicrobubble-enhanced ultrasound imaging, and the more recent advances in
super-resolution imaging and molecular imaging, have shown great promise
in a wide range of clinical applications. However, a number of challenges
still exist during their clinical translations. For instance, in ultrasound superresolution imaging, the in vivo lifetime of microbubble is normally less than
5 minutes and the concentration of microbubbles in vivo is difficult to adjust
after the injection. Nanodroplet, as a condensed version of microbubble, has
a number of advantages in super-resolution imaging. Firstly, nanodroplet
have a longer lifetime compared to microbubbles. This enables less contrast
agents injected and a longer scanning time. Second, these nanodroplets can
be controlled to be activated to provide the contrast signals both spatially and
temporally on-demand. This potentially provides more flexibility during the
ultrasound scanning. In the context of molecular imaging, the nano-size of
droplets potentially allows extravasation into cancerous tissue due to its
leaky vasculature and enhanced permeability and retention effects where
microbubbles cannot. In the field of photoacoustic imaging, dye-coated
nanodroplet was developed to provide a significant contrast enhancement in
photoacoustic imaging upon the optical activation where microbubbles also
cannot be used.
Among a variety of research efforts on facilitating the pre-clinical
translation, this work focuses on the applications of nanodroplets in
ultrasound and photoacoustic super-resolution and molecular imaging. There
are four main scientific contribution as follows. First, the feasibility of using low-boiling-point nanodroplets to perform
ultrasound super-resolution imaging was investigated. ‘Acoustic wave
sparsely activated localization microscopy (AWSALM)’, an acoustic
counterpart of photo-activated localization microscopy (PALM), is
developed to super-resolve structures which cannot be resolved by
conventional B-mode imaging. AWSALM utilizes acoustic waves to
sparsely and stochastically activate decafluorobutane nanodroplets by
acoustic vaporization and to simultaneously deactivate the existing
vaporized nanodroplets via acoustic destruction. This technique is less
dependent on flow and does not require a low concentration of contrast
agents, as is required by current ultrasound super resolution techniques.
Acoustic activation and deactivation can be controlled by adjusting the
acoustic pressure, which remains well within the FDA approved safety
range. This study shows the promise of a flow and contrast agent
concentration less dependent super-resolution ultrasound technique which
has potential to be faster and go beyond vascular imaging.
Second, to achieve super-resolved imaging frames with sub-second
temporal resolution, fast acoustic wave sparsely activated localization
microscopy (fast-AWSALM) was developed by using low-boiling-point
octafluoropropane nanodroplets and high frame rate plane waves for
activation, destruction, as well as imaging. The effects of the temperature
and mechanical index on fast-AWSALM was investigated. The contrast
signals were quantified as a function of acquisition time. The final results
showed two orders of magnitude faster than the reported localization-based
ultrasound super-resolution techniques, under a non-flow/very slow flow
situations. Just as in AWSALM, fast-AWSALM is less dependent on flow,
as is required by current microbubble based ultrasound super resolution techniques. This study shows the promise of fast-AWSALM, a superresolution ultrasound technique using nanodroplets, which can
generate super-resolution images in milli-seconds.
Third, to have a better design of targeted nanodroplets for imaging and
therapeutic applications, the size and acoustic response of targeted
nanodroplets under high-frame-rate ultrasound and optical imaging was
investigated. A flow velocity mapping technique, Stokes’ theory and optical
microscopy were used to estimate the size of both floating and attached
vaporized nanodroplets immediately after activation. The floating vaporized
nanodroplets were on average more than seven times larger than vaporized
nanodroplets attached to the cells. The results also indicated that the acoustic
signal of vaporized FR-targeted-nanodroplets persisted after activation, with
70% of the acoustic signals still present 1 s after activation, compared with
the vaporized NT-nanodroplets, for which only 40% of the acoustic signal
remained. The optical microscopic images revealed on average six times
more vaporized FR-targeted-nanodroplets generated with a wider range of
diameters (from 4 to 68mm) that were still attached to the cells, compared
with vaporized NT-nanodroplets (from 1 to 7mm) with non-specific binding
after activation. The mean size of attached vaporized FR-targetednanodroplets was on average about threefold larger than that of attached
vaporized NT-nanodroplets. Taking advantage of high-frame-rate contrastenhanced ultrasound and optical microscopy, this study offers an improved
understanding of the vaporization of the targeted nanodroplets in terms of
their size and acoustic response in comparison with NT-nanodroplets. Such
understanding would help in the design of optimized methodology for
imaging and therapeutic applications. Fourth, the feasibility of using dye-coated nanodroplets to perform
photoacoustic super-resolution imaging was investigated. A photoacoustic
super-resolution imaging technique was developed through imaging the
activation of Cyanine 7.5-coated phase-change nanodroplets using a
preclinical photoacoustic imaging system and localizing the activated
droplets. As a proof-of-concept experiment, photoacoustic images of
microfluidic channels were obtained with a cylindrically focused curvedarray, while dye-coated nanodroplets flowed through the channels.
Experimental results showed that super-resolution images can resolve
structures which cannot be resolved by conventional beamformed images in
vitro. The results also showed that the dye-coated phase-change
nanodroplets can be optically activated in vivo and the activation signals can
be separated from the image background by applying singular value
decomposition filtering, and be used for further super-localization
processing. Nanodroplets offer better biocompatibility, as well as more
flexible and controllable droplet activation rates, with potential for superresolution imaging without flow and of extravascular targets, compared to
contrast agents used in existing localization-based photoacoustic superresolution imaging techniques.Open Acces
Super-resolution ultrasound image filtering with machine-learing to reduce the localization error
No full textLocalization-based super-resolution imaging requires accurate detection of spatially isolated microbubbles. The reason for this requirement is that interfering or overlapping signals resulting from multiple microbubbles within the resolution limit can cause position errors. In addition to this, noise and artefacts (e.g. residual tissue signal after tissue-microbubble separation) further reduce the quality and hence the spatial resolution in SR imaging. Therefore, correctly identifying the echoes as noise, single microbubble, multiple microbubbles, or artefact is important.In this study, the use of fast classification methods for identification and rejection of non-single microbubble echoes were demonstrated. Most commonly used supervised classification methods, including Decision Trees, Discriminant Analysis, Logistic Regression, Support Vector Machine, Ensembles, k-Nearest Neighbors, and Naive Bayes, were implemented for filtering artefacts and noise in super-resolution ultrasound images. Results showed that the Ensemble method, explicitly designed to deal with unbalanced data, achieved the best result since most of the localized events are true microbubbles, which is typical for super-resolution imaging datasets.</p
Ultrasonic phased array device for acoustic imaging in air
No full textThe acoustic imaging technology is widely used for medical purposes and underwater imaging. In this work, an ultrasonic phased array device is developed by using piezoelectric transducers to provide autonomous navigation for robots and mobility aid for visually impaired people. To perform acoustic imaging, two different linear transducer arrays are composed with phase-delay focusing phenomenon in order to detect proximate objects with no mechanical scanning. The requirement of half wavelength spacing can not be satisfied between elements, because of using general purpose transducers. The transmitter array is formed by aligning the transducers with minimum spacing between them, which is 2.11 times of the wavelength. This placement strategy leads to the occurrence of unwanted grating lobes in the array response. To eliminate these grating lobes, the receiver array is formed with a different spacing between each transducer. By forming the receiver array and the transmitter array non-identical, the directivity pattern for both arrays become different. The off-alignment between two arrays causes the grating lobes to appear at different places. Since the overall gain of the system is the product of receiver gain and transmitter gain, the grating lobes diminish for the overall system. The developed phased array device can transmit/receive ultrasonic waves to/from the arbitrary front directions using electronic sector scanning circuits. A detailed scan can be performed to detect the presence of an object or distinguish different objects
Use of chirps in medical ultrasound imaging
No full textA "chirp" is a frequency modulated signal widely used in ultrasound imaging to increase the signal-to-noise ratio and penetration depth. In medical ultrasound imaging, resolution and penetration are two major criteria that
are inversely proportional. Because of this inverse relation, short duration pulses cannot achieve a high resolution with good penetration. The reasons
for this trade-off are the decrease in signal energy due to shorter pulse duration and the attenuation in tissue, which increases with the excitation frequency. The chirp coded excitation however can increase the total transmitted energy using longer pulse durations, while the resolution can be recovered by decoding on receive. Therefore, chirp signals offer potential advantages over single carrier short duration pulses for medical imaging.
This work addresses the possible problems encountered in medical ultrasound imaging with chirps and offers new solutions to these problems in terms of signal processing. These proposed solutions are then applied to three major categories of medical ultrasound imaging; hard-tissue ultrasound imaging, soft-tissue ultrasound imaging and contrast-enhanced ultrasound imaging.
The application of coded excitation in medical ultrasound imaging is the main motivation behind this work. Therefore, the concepts of frequency modulation and matched filtering are introduced first, and ultrasound specicific problems for pulse compression of chirps are discussed. Examples are
given on specific applications and circumstances, where the performance of the traditional pulse compression techniques drops significantly.
Alternate methods of pulse compression and filtering of frequency modulated chirps using the Fractional Fourier transform (FrFT) and the Fan Chirp transform (FChT) are presented. Rather than restricting the chirp analysis in the time or frequency domain; these proposed methods transform the signal of interest into a new domain, which is more suitable to analyse frequency modulated chirps
Ultrasonic Phased Array Device for Real-Time Acoustic Imaging in Air
No full textA real-time acoustic imaging system is developed as a prototype electronic travel aid (ETA) device. The design is implemented on a field programmable gate array (FPGA). A 6 channel transmit and 4 channel receive digital beamforming algorithm with dynamic focusing is accommodated in a FPGA. The developed system consists of a FPGA, pulser and receiver circuitry and separate transmitter and receiver arrays, which are constructed by using commercially available transducers. The transducer elements have a physical dimension of 1.9 wavelengths and a half-power beamwidth of 43◦ at 40.8 kHz center frequency. The transmitter array is formed by aligning the transducers with minimum spacing between the elements, which is 2 wavelengths. Obviously, more than one wavelength inter-element spacing leads to the occurrence of grating lobes in the array response and decreases the Field of View (FOV) below the half-power beamwidth of transducers. To extend the FOV and eliminate the grating lobe, the receiver array is formed with 3 wavelength inter-element spacing. The non identical element spacing makes the grating lobes of transmitter and receiver array to appear at different places. The described placement strategy and the functionality of the system is tested with several experiments. The results of these experiments prove the grating lobe suppression capability of the applied placement strategy
Ultrasonic Phased Array Device for Acoustic Imaging in Air
No full textAn ultrasonic phased array device is developed to
provide mobility aid for visually impaired people. To perform
acoustic imaging, two different linear transducer arrays are
composed using commercially available transducers for ranging
applications. The transmitter and receiver arrays are formed
with six and four transducer elements, respectively. Individual
transducer elements are discrete components with a radius of
1.9 wavelengths and a half-power beamwidth of 43± at 40.8 kHz
center frequency. The transmitter array is formed by aligning the
transducers with minimum spacing between the elements. Even
this placement leads to the occurrence of unwanted grating lobes
in the array response and decreases the Field of View to 30±. To
eliminate these grating lobes, the elements of the receiver array
are placed with a different spacing. Forming the receiver and
transmitter arrays with non-identical element spacing causes the
grating lobes to appear at different places. Since the response of
the overall system is the product of the directivity patterns of
receiver and transmitter arrays, the grating lobes diminish for
the overall system and the Field of View increases
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